1. Field of the Invention
The present invention relates to a manufacturing method of semiconductor device having thin film transistor, particularly to a technique for forming crystalline semiconductor film, which comprises active layer of thin film transistor.
2. Related Art
As a forming method of active layer on thin film transistor (Thin Film Transistor: hereinafter, referred to as TFT), a technique, in which an amorphous semiconductor film is formed on a substrate having an insulation surface, and then, crystallize the same in a manner of laser annealing or heat annealing, has been developed.
The laser annealing is known as a crystallization technique, in which a high energy is given to an amorphous semiconductor film only without allowing the temperature of a glass substrate to rise too high, and thereby, the amorphous semiconductor film is crystallized. Particularly, excimer laser is a typical laser, which oscillates short-wave length light of 400 nm or less in wavelength, has been used since the laser annealing was developed. The laser annealing is carried out in such a manner that a laser beam is processed by means of an optical system so as to be shaped into a spot-like form or a linear form at a surface to be irradiated, and the surface to be irradiated on the substrate is scanned by the processed laser beam; i.e., irradiation position of the laser beam is shifted with respect to the surface to be irradiated.
However, the crystalline semiconductor film, which is prepared by means of the laser annealing, comprises an aggregate of a plurality of crystal grain (ordinary crystal grain size, which is prepared by means of a conventional excimer laser crystallization, is approximately 0.1-0.5 xcexcm), and, the position and the size of the crystal grain are not even.
As for the TFT, which is prepared on a glass substrate, in order to isolate elements, since the crystalline semiconductor film is formed being separated into island-like patterns, it was impossible to form crystal grains at specified positions and sizes. Accordingly, it was almost impossible to form channel-forming areas with a monocrystal semiconductor while eliminating the influences of the crystal grain boundary.
The interface (crystal grain boundary) of the crystal grain is an area where the translational symmetry of the crystal is decayed. It is known that, due to the influence of the recombination center or trapping center of the carrier, or the influence of the potential barrier in the crystal grain boundary caused from the crystal defect or the like, the current transport characteristics of the carrier is decreased, and as a result, the OFF-current increases in the TFT.
A technique called as super lateral growth, by which, compared to the crystal grain size via conventional excimer laser crystallization, a larger grain size can be formed, is known. A detailed description of the technique is disclosed in xe2x80x9cOn the super lateral growth phenomenon observed in excimer laser-induced crystallization of thin Si films, James S. Im. and H. J. Kim, Appl. Phys. Lett. 64(17), Apr. 25, 1996, pp2303-2305xe2x80x9d.
In the super lateral growth, a portion, where the semiconductor is melted completely due to the irradiation of a laser beam, and a portion where the solid-phase semiconductor area remains, are formed, and then, the crystal growth begins around the solid-phase semiconductor area as the crystal nucleus. Since it takes a certain period of time for nucleation to take place in the completely melted area, during the period of time until the nucleation takes place in the completely melted area, the crystal grows around the above-described solid-phase semiconductor area as the crystal nucleus in the horizontal direction (hereinafter, referred to as lateral direction) with respect to the film surface of the above-described semiconductor. Therefore, the crystal grain grows up to a length as long as several tens of times of the film thickness. For example, with respect to the silicon film thickness of 60 nm, a lateral crystal growth of 1 xcexcm-2 xcexcm in length takes place. Hereinafter, the phenomenon will be referred to as super lateral growth.
In the case of the above-described super lateral growth, although a relatively large crystal grain can be obtained, the energy intensity area of the laser beam, where the super lateral growth is obtained, is much stronger than the intensity that is used in ordinary excimer laser crystallization. Also, the range of the energy intensity area is extremely narrow. From the viewpoint of the position control of the crystal grain, it is impossible to control the position where a large crystal grain is obtained. Further, the area other than that of the large crystal grain is the microcrystal area where countless nucleation has taken place, or the amorphous area; the size of the crystals is not even and the roughness of the crystal surface is extremely large.
Accordingly, the irradiation condition, which is generally used in manufacturing of semiconductor devices, is the condition where even crystal grain size of approximately 0.1 xcexcm-0.5 xcexcm can be obtained.
Further, according to xe2x80x9cSequential lateral solidification of thin silicon films on SiO2, Robert S. Sposili and James S. Im, Appl. Phys. Lett. 69(19), Nov. 4, 1996, pp2864-2866xe2x80x9d, James S. Im et al. disclosed a Sequential Lateral Solidification method (hereinafter, referred to as SLS method), in which, by controlling artificially, the super lateral growth can be obtained at a desired position. According to the SLS method, an excimer laser beam of pulse oscillation is irradiated to a material via a slit-like mask. According to the SLS method, the crystallization is carried out while the relative position between the material and the laser beam is displaced, at every shot, by a distance (approximately 0.75 xcexcm), which is roughly equivalent to the length of the crystal formed via the super lateral growth; thereby, the crystal is allowed to grow by means of artificially controlled super lateral growth.
As described above, by using the SLS method it is possible to prepare crystal grains at desired positions under artificially controlled conditions, in a manner of the super lateral growth. However, the SLS method has the following problems as described below.
First of all, the problem is that the substrate processing efficiency (throughput) is insufficient. As described previously, in the SLS method, the crystallization distance per shot of laser beam is approximately 1 xcexcm. Accordingly, it is necessary that the relative shift distance (feed pitch) of the laser beam between the beam spot on the material surface and the material substrate is 1 xcexcm or less. In the conditions adopted in the ordinary laser crystallization using a pulse oscillation excimer laser, feed pitch per shot of laser beam is several 10 xcexcm or more. Needless to say, under such conditions, the crystal peculiar to the SLS method can not be prepared. In the SLS method, although a pulse oscillation XeCl excimer laser is used, the maximum oscillation frequency of the pulse oscillation XeCl excimer laser is 300 Hz. Under such conditions, only the crystallization area of maximum 300 xcexcm or so is processed in distance in the scan direction of the laser beam. At the processing speed as described above, in the case of a large size substrate such as, for example, 600 mmxc3x97720 mm in dimension, with the SLS method, it takes an extremely long period of time to process one sheet of substrate.
The fact that it takes a long processing time per sheet of substrate is not only the problem of time and cost. That is to say, practically, in the case of crystallization of an amorphous semiconductor film, the surface processing thereof is critical. For example, in the case that laser irradiation is carried out after removing natural oxide film with dilute hydrofluoric acid or the like as a pre-processing, in the surface of the substrate, compared to the area where is subjected to the laser irradiation at the first, there is a possibility that natural oxide film grows again in the area where is subjected to the laser irradiation at the last. In this case, amount of Carbon, Oxygen, Nitrogen, or amount of contamination impurities such as Boron or the like, which is taken in the finished crystal, may vary within the surface of the substrate resulting in an unevenness of the transistor characteristics within the surface of the substrate.
Secondly, such a problem that the optical system tends to be complicated remains in the conventional SLS method. It is necessary to incorporate a mask, which processes slit-like the configuration of the laser beam power at the substrate surface, into the optical system. Ordinarily, film thickness of an active layer silicon, which is used for polycrystalline silicon thin film transistor, is several tens nm or more. When a pulse oscillation excimer laser is used, the laser energy density necessary for the laser crystallization is at least 200 mJ/cm2 (as a typical example, for an amorphous silicon film of 50 nm, approximately 40 mJ/cm2 with an XeCl excimer laser of 30 n sec pulse width). In the SLS method, there is a super lateral growth condition, which is the optimum for a further slightly stronger energy density area. It is difficult to prepare a slit-like form mask, which is capable of enduring such strong laser energy density. In the case of the mask of metal material, being subjected to the pulse laser beam irradiation of a strong energy density, the temperature of local film is raised and cooled down rapidly. As a result, it may cause a peeling or decay of configuration of a minute pattern due to a long period of use (as for photo lithography for resist exposure, although a hard mask material such as chromium or the like is used, since incomparably weaker energy density than the laser energy density necessary for silicon crystallization is used, there is no problem such as peeling or a decay of configuration of the minute pattern). As described above, there is such a factor that optical system becomes complicated resulting in a difficulty of maintenance of an apparatus in the conventional SLS method. Further, in order to carry out the super lateral growth, it is necessary to make the spatial beam power profile of the laser beam sharp (to eliminate attenuation area of the optical power between the irradiation area and the non-irradiation area of the laser beam). In the conventional SLS method, since the beam necessary for the super lateral growth cannot be condensed using the ordinarily optical system only, the excimer laser is used. Accordingly, it is understood that a slit-like mask is required to shield the laser beam partially.
The object of the invention is to solve the above-described problem, and further, to increase the positional control of crystal grains in accordance with the layout of the TFT, and simultaneously, to increase the processing speed of the crystallization process. More particularly, it is an object of the invention to provide a manufacturing method of semiconductor devices, which is capable of forming large size crystal grains successively in a manner of super lateral growth under an artificial control, and capable of increasing the substrate processing efficiency in the laser crystallization process.
Further, the invention provides a manufacturing method of semiconductor devices which is capable of forming large size crystal grains successively in a manner of super lateral growth under an artificial control, and capable of increasing the substrate processing efficiency in the laser crystallization process, as well as to provide a manufacturing method using a convenient laser irradiation method which does not need to incorporate a mask, which processes the configuration of the laser beam power into a slit-like shape on the substrate surface, into an optical system unlike the conventional SLS method.
A laser irradiator applied to the invention includes a first means for controlling the irradiation position of laser beam with respect to an object to be processed (substrate and thin film formed on the substrate); a second means (laser oscillator) for oscillating the laser beam; a third means (optical system) for processing the laser beam; and a fourth means for controlling the oscillation of the second means, and for controlling the first means so that the beam spot of the laser beam, which is processed by the third means, covers the specified position in accordance with data (pattern information) of photomask configuration.
As the first means for controlling irradiation position of the laser beam with respect to the object to be processed, two methods are available. One of the methods is a method in which the position of the object to be processed placed on the stage is changed by driving the stage by means of a stage controller. The other one is a method in which the irradiation position of the laser beam spot is shifted using a laser optical system in a state that the substrate position is fixed. In the invention, any one of the above-described two methods is applicable; and a method, in which the above-described two methods are combined, is also applicable.
The position specified in accordance with the data (pattern information) of photomask configuration is the portion in a semiconductor film, which becomes a channel area, a source area or a drain area in the thin film transistor, and is obtained by carrying out patterning processing by means of photo lithography technique on an island-like semiconductor layer B after crystallization.
Also, in the invention, before the laser beam irradiation, it is necessary to subject the semiconductor to an patterning processing on an island-like semiconductor film A, which is specific area including an active layer forming area comprised of thin film transistor by means of photo lithography technique, and to form markers on parts of semiconductor film. The marker is necessary to realize the above-described fourth means. Further, the island-like semiconductor layer A is slightly larger than the island-like semiconductor layer B. FIG. 2 shows a portion 500 as an example of the island-like semiconductor layer A, and a portion 501 as an example of the island-like semiconductor layer B. That is to say, it is a mode that the island-like semiconductor layer B, which will finally become a channel area, a source area and a drain area of the transistor is included in the island-like semiconductor layer A.
Using the laser irradiator, which has the above-described first means to fourth means, the island-like semiconductor layer A is crystallized. At this time, using the fourth means, a part which is left as island-like semiconductor layer B on the substrate after patterning processing in the semiconductor film, which has been formed on the insulation surface, is comprehended in accordance with the data of the photomask configuration. And, the laser beam is irradiated selectively to the island-like semiconductor layer A to crystallize the area using the marker as the positional reference.
Next, the periphery portion of the island-like semiconductor layer A is subjected to a etching by means of photo lithography technique, and the island-like semiconductor layer B is subjected to a patterning processing. The island-like semiconductor layer B is used as the active layer of the transistor.
As described above, according to the invention, the laser beam is irradiated in such a manner that, not the entire semiconductor in the substrate surface is scanned by the laser beam, but at least the minimum indispensable portion thereof is crystallized. That is to say, by carrying out the patterning processing on the island-like semiconductor layer B after the semiconductor has been crystallized, it is possible to reduce the time necessary for irradiating the laser beam to the portion to be removed. Owing to this, it is possible to reduce the time necessary for laser crystallization and to increase the processing speed of the substrate.
It is necessary that, after forming the island-like semiconductor layer A, the laser beam irradiation is carried out; and after that, the island-like semiconductor layer B, which will become the active layer of the transistor, is formed, to ensure the positional control of the crystal grain in accordance with the layout of the TFT.
By applying the above-described constitution to the conventional SLS method, the problem in the conventional SLS method that substrate processing efficiency (throughput) is insufficient, is solved. Also, a means for ensuring the positional control of the crystal grain in accordance with the layout of the TFT is obtained.
Further, according to the invention, the time necessary for laser crystallization can be reduced. And further, a method that increases the processing speed of the substrate and a method that ensures the positional control of the crystal grain in accordance the layout of the TFT are obtained. Furthermore, unlike the conventional SLS method, the simple method that does not need to incorporate a mask for processing the configuration of laser beam power at the surface of the substrate into the optical system is obtained.
In order to obtain the super lateral growth, it is necessary to change the spatial energy distribution of the laser beam sharply in the direction of the lateral crystal growth (i.e., the direction in which the solid-liquid interface of the semiconductor film after laser irradiation). That is to say, it is necessary to eliminate the attenuation area width of optical power, which resides between the irradiation area and the non-irradiation area of the laser beam, as much as possible. The attenuation area width capable of obtaining satisfactory super lateral growth is defined as below; i.e., the attenuation area width from the peak position of the optical power to a point where the power decreases to 50% is 10 xcexcm or less.
In the conventional SLS method, since an excimer laser is used, the density necessary for the super lateral growth cannot be obtained by the ordinary optical system only. Accordingly, it is understandable that a slit-like mask is necessary to be used to shield the laser beam partially.
The light source of the above-described laser beam is a system that irradiates the second harmonic (or, third harmonic or fourth harmonic) of the solid-state laser oscillator of pulse oscillation. Compared to the excimer laser, in the solid state laser, as the spreading angle of the output laser beam is small, owing to the laser constitution, with a cylindrical lens only that is used as ordinary optical system lens, it is possible to condense the beam into a spatial beam power profile of the laser beam that is the optimum for the super lateral growth.
In order to increase the substrate processing efficiency, it is desired to select a repeat frequency and a feed pitch that is the optimum for the SLS method. The conditions for that will be described below. The word xe2x80x9cfeed pitchxe2x80x9d means shift distance of the substrate stage per pulse of the laser beam. In the SLS method, since the distance of the super lateral growth per shot is limited to a specific length, by enlarging the feed pitch only, the substrate processing efficiency cannot be increased. When the feed pitch is increased, it is necessary to increase also the repeat frequency of the laser beam accordingly. The XeCl excimer laser used in the conventional SLS method is maximum 300 Hz. On the other hand, the solid-state laser oscillator of pulse oscillation can increase the repeat frequency to the maximum several MHz. Accordingly, compared to the conventional SLS method, the processing capacity can be largely increased by driving the solid-state laser oscillator of pulse oscillation to irradiate at a repeat frequency. The upper limit of the repeat frequency can be determined within a range that ensures the energy density necessary for the super lateral growth at every shot of laser beam. The upper limit depends on the maximum output of the solid-state laser oscillator of pulse oscillation. (Since, if the other conditions are the same, when the frequency is increased, the energy density at every laser pulse is reduced.)
Further, in the solid-state laser oscillator, not the conventional flash lamp excitation but semiconductor laser excitation solid-state laser oscillator increases the stability of the laser beam energy largely. As a result, it is possible to form a semiconductor of which crystallinity fluctuation is smaller. Accordingly, it is possible to manufacture semiconductor device of which fluctuation in TFT characteristics is smaller.
Further, compared to the excimer laser irradiator, the solid-state laser oscillator is superior in maintainability.
Furthermore, compared to the excimer laser irradiator, the pulse width of the solid-state laser is longer. Since the time for melting and crystallizing becomes longer by adopting the longer pulse width, larger crystal grain can be formed.
Still further, by elongating the pulse width, it is possible to reduce the temperature difference between the semiconductor surface, where is to be irradiated by the laser, and the interface (for example, base film) between the semiconductor film and the film abutting to the bottom face thereof. As described above, by reducing the temperature difference, the core generating speed becomes slower.
FIG. 14 shows a result of simulation of the relationship between the pulse width and the base film temperature at crystallization. When the maximum reached temperature of the semiconductor surface is 1500K, 2000K and 2500K respectively, the temperature of the base film tends to become higher as the pulse width is longer, and then, the level of the temperature becomes fixed. Also, when the pulse width is larger than 50 ns, and preferably, larger than 100 ns, it is possible to reduce the temperature difference between the base film temperature and the maximum reached temperature of the interface, it is possible to make the core generating speed more slowly.
The following table show a comparison between the XeCl gas laser irradiator and the Nd:YLF solid-state laser irradiator in the SLS method.
Owing to the constitution as described above, it is possible to provide a manufacturing method of semiconductor devices which is capable of forming large size crystal grains successively in a manner of super lateral growth under an artificial control, and capable of increasing the substrate processing efficiency in the laser crystallization process, as well as to provide a manufacturing method using a convenient laser irradiation method which does not need to incorporate a mask, which processes the configuration of the laser beam power into a slit-like shape on the substrate surface, into an optical system unlike the conventional SLS method.
The word xe2x80x9csemiconductor devicexe2x80x9d in the invention includes every apparatus that is capable of functioning by using the semiconductor characteristics (for example, electronic device represented by liquid crystal display and electronic apparatus equipped with the electronic device as a part thereof).